As shown in FIG. 6, in a drive mechanism of this type of a scroll compressor in general, a shaft 3 is supported in a housing 1 by a bearing 2, a boss portion is formed on the central portion of an orbiting scroll end plate 5a opposite a fixed scroll 4, and the shaft 3 is coupled with an orbiting bushing 7 by affixing on the boss portion an orbiting bearing 6 in which the orbiting bushing 7 is inserted, thereby enabling orbiting motion of an orbiting scroll 5 relative to the fixed scroll 4 by the rotation of the shaft 3. Also, a suction port 8 for sucking a refrigerant as a working fluid is provided on the housing 1 on the side of the low pressure chamber, and a discharge port 10 for discharging compressed refrigerant is provided on a housing 9 on the high pressure side.
In this configuration, a rotation preventing mechanism forces the orbiting scroll 5 to orbit while hampering its rotation. The mechanism includes first engagement means movable between an elliptical Oldham's ring 11 shown in FIGS. 7(a) and 7(b) and the orbiting scroll 5 only in a first radial direction (direction of the X-axis), and second engagement means movable between the housing 1 and the Oldham's ring 11 only in a second radial direction (direction of the Y-axis) perpendicular to the first radial direction.
The first engagement means comprises a pair of key ways (first key ways) provided on the orbiting scroll end plate 5a in the first radial direction and a pair of first keys 11a provided on the Oldham's ring 11, each of the first keys respectively fitting with each of the first key ways and slidable along the first key ways and a thrust plate 12. The second engagement means comprises a pair of key ways (second key ways) provided on the housing 1 in a second radial direction and a pair of second keys 11b provided on the Oldham's ring 11, each of the second keys respectively fitting with each of the second key ways and slidable along the second key ways and the thrust plate 12. The mechanism is configured in a manner such that the rotation of the orbiting scroll 5 is hampered by the first engagement means and the second engagement means while its orbiting motion is allowed. As another example, there is also a configuration in which a circular ring is used in place of the elliptical ring used in the ring portion of the Oldham's ring 11.
However, in the above described construction of the rotation hampering mechanism, the Oldham's ring 11 is disposed in a manner such that each of the key pairs 11a and 11b are opposingly disposed respectively on the x- and y-axes (in the first and second radial directions) of an elliptical ring having major and minor axes. As a result, it suffered a problem of the key portions being damaged because reciprocating force of inertia of the Oldham's ring 11 increases during a high-speed high-pressure operation thus imposing an excessive load on the key portions. In order to secure a sufficient strength of the key portions, it is necessary to make the reciprocating force of inertia of the Oldham's ring 11 small. For this purpose, it is necessary to widen the ring width, ring diameter or the key widths of the Oldham's ring 11, which will result in an increase in the size of the scroll compressor as a whole thus harming salability of the product.